World's 'oldest water' to be found in the deep blue sea

Those trying to communicate across the seas using a message in a bottle had better hope it doesn't get caught in a patch of stagnant water in the north Pacific that could mean it takes 2000 years to resurface.

The so-called shadow zone of very slow overturning circulation - stretching about 6000 kilometres west to east, 2000 kilometres north to south, and lying 1-2.5 kilometres below the ocean's surface - is home to the world's oldest water, researchers including several based at the University of NSW have found.

The rough location of the water with unusually extended periods between contact with the atmosphere has been known for some time. The new research, pushed recently in Nature, helps explain the processes involved that keep the water from circulating back to the surface for 1500 or perhaps even 2000 years.

"Our main advance is actually to understand how and why this water is so old," said Ryan Holmes, from the ARC Centre of Excellence for Climate System Science, and one of the paper's authors.

"We've developed a theory that explains all our observations - it's simply dependent on the shape of the sea floor," Dr Holmes said.

The Atlantic Ocean lacks sufficient conditions to have a shadow zone of similar longevity. While the Indian Ocean has a shadow zone, its relative size and the fact it lies closer to the start of the "young water" sourced from Antarctica means its patch of stagnant water is of a lesser vintage than the north Pacific's.

The overturning process begins with ice forming on the ocean surface around Antarctic, expelling its salt. The denser cold water then sinks, sliding down the continental margin and flowing northwards.

The process also helps ventilate the world's oceans, since Antarctic waters have a relatively high oxygen content compared with other seas.

The relatively stagnant layer of north Pacific water is partly due to the depth of the ocean, giving it sufficient distance from either the surface or the sea floor with its geothermal activity that would promote mixing.

The relatively low oxygenation of the shadow zones mean marine life may fare less well than elsewhere.

"It's not a zone of very flourishing life but that doesn't mean it's a dead zone," said Casimir de Lavergne, the paper's lead author from UNSW's School of Mathematics and Statistics.

The researchers say their work could help other scientists understand more about the capacity of oceans to continue to absorb the great bulk of the additional heat trapped by the additional greenhouse gases being driven higher by humans burning more fossil fuels and clearing land.

Oceans also absorb much of the extra carbon emitted by those actions.

"[W]hile the research may have answered one question about deep ocean water, it has also opened doors to answer more questions that may directly relate to the future impacts of climate change and the ecology of our major oceans," Dr Holmes said.